Histone deacetylases inhibitors: conjugation to other anti-tumour pharmacophores provides novel tools for cancer treatment

Abstract

Histone deacetylases (HDACs) are involved in the removal of acetyl groups from intracellular proteins. The catalytic activity of HDACs plays a major role in numerous biological processes, including cell-cycle regulation, cell proliferation and apoptosis. Importantly, tumour development and progression have been associated with altered expression and mutations of genes that encode members of the HDAC family. This family comprises at least 18 enzymes that are responsible for the post-translational deacetylation of several histone and non-histone proteins. HDACs hold a place among the most promising therapeutic targets for the treatment of cancer and there are growing efforts to optimise HDAC inhibition therapy. The authors believe that there is the need for an innovative pharmacological strategy, if the field wants to significantly ameliorate the current shortcomings of the current cancer therapies, in particular, perhaps a strategy that focuses on developing single HDAC inhibitor-based compounds, which can modulate the functions of additional intracellular oncogenic targets via conjugation to other anti-tumour pharmacophores.

Keywords::

• anti-tumour pharmacophore
• cancer treatment
• conjugation
• histone deacetylase inhibitor
• multi-targeted therapy

1. Introduction

Deacetylation of histone and non-histone proteins has been recognised as a key regulatory mechanism in cellular biology. The family of enzymes that are responsible for removing acetyl groups from several intracellular protein substrates are known as histone deacetylases (HDACs). Given their prominent role in many vital cellular processes including control of gene expression, chromatin conformation, protein–DNA interaction, cellular differentiation, growth arrest and apoptosis, HDACs have emerged as promising new molecular targets for cancer therapy.

At present, the majority of HDAC inhibitors (HDACi) are non-selective, targeting more than 1 of the 11 zinc-dependent HDAC metalloenzymes. Acting against several HDAC family members potentially results in the numerous side effects that pan-HDACi exhibit in the clinic. This non-specificity of current HDACi is limiting their clinical potential and presents hurdles towards effective and safe HDAC inhibition. The two Food and Drug Administration (FDA) approved HDACi, vorinostat (SAHA) and romidepsin (FK228), have proven successful against blood cancers; nevertheless, SAHA, FK228 and HDACi in clinical testing have demonstrated a preferential clinical efficacy in haematological malignancies and little or no anticancer activity against solid tumours. To date, there is lack of convincing clinical evidence of activity of HDACi in solid malignancies. All these shortcomings of current HDACi as single agents for the treatment of cancer has prompted efforts into the design and synthesis of HDACi with novel features that will overcome the limitations of this class of therapeutics and sufficiently elicit their potent anti-proliferative and pro-apoptotic activities against both haematological and solid tumour malignancies.

Future steps in cancer drug development require sophisticated and rational pharmacological targeting of HDACs that will yield therapeutic effects with superior safety for diverse tumour entities. Among such new approaches are rational combinations of HDACi with currently approved chemotherapies and isoform-selective HDAC inhibition [1,2]. Combination therapies act by directly blocking several oncogenic signalling pathways and creating synergistic anti-tumour effects; however, combining HDACi with other cancer therapeutics may be complicated by the production of adverse effects related to pharmacokinetics, toxicity and patient-compliance. Isoform-selective HDACi may provide a better clinical benefit than pan-HDACi, while reducing or eliminating undesirable effects [2-5]. This strategy focuses on targeting selectively individual HDAC isoforms that are already known or suspected to contribute to cancer. Such new isozyme-selective inhibitors are expected to lead to better clinical efficacy in solid tumours by providing better exposure and allowing for higher dose levels of the drug. Moreover, considering that this class of enzymes function within cells as part of multi-protein complexes, researchers may pursue development of complex-specific HDACi that could provide superior selectivity [6]. Our increasing insights into the role of particular HDAC members in the formation and maintenance of many cancers are expected to yield highly potent and selective HDACi for clinical development. Additional approaches that hold promise include localised administration of HDACi into the tumour tissue and tissue-specific HDACi delivery conferred by an appended surface recognition cap group [7]. An enticing new paradigm in the development of anticancer agents aims to design single molecules that are equipped with inhibitory activities for multiple oncogenic targets [8]. This emerging approach holds a lot of potential for the treatment of many cancers. Currently, there exist a few examples of this novel class of HDACi-based hybrid compounds.

Researchers designed and synthesised colchicine–SAHA hybrids based on the synergistic anti-tumour effect of tubulin inhibitors and HDACi. It is the first that reported design of molecules which are dual inhibitors of tubulin and HDAC [9]. Inspired by the synergistic effects of trichostatin A and 1α,25-dihydroxyvitamin D(3) (1α,25(OH)(2)D(3) or 1,25D), investigators developed a class of therapeutic agents that are hybrids of 1,25D and HDACi. They achieved in combining nuclear receptor agonism with HDACi activity in a single stable entity. These molecules provide excellent proof-of-principle for the concept of single multivalent anticancer compounds and possess enhanced cytostatic/cytotoxic activity. Their next steps include optimisation of the potencies of both the vitamin D receptor agonist and HDACi activities of these hybrids [10]. Taking into account that receptor tyrosine kinase (RTK) inhibitors have become important chemotherapy drugs for a variety of cancers, although their effectiveness is often hindered by poor response rates and acquired drug resistance, recent studies explored a novel type of multi-targeted agents, RTK/HDAC dual inhibitors. These compounds can simultaneously inhibit HDACs as well as RTK, creating a potent HDACi–HER2i hybrid. Pharmaceutical studies revealed the potential ability of such drugs to overcome tumour recurrence and drug resistance [11].

Along the same lines, another group of researchers chose to integrate into one compound the crucial structural elements required to inhibit epidermal growth factor receptor (EGFR)/HER2 as well as HDACs. The resulting hybrid drug, CUDC-101, displayed potent anti-proliferative and pro-apoptotic activities against cultured and implanted tumour cells that were sensitive or resistant to several approved single-targeted drugs. The potency achieved by this specific multi-targeted inhibitor surpassed that of combination therapies with HDAC and EGFR/HER2 inhibitors. This study showed that CUDC-101 has the potential to dramatically improve the treatment of heterogeneous and drug-resistant tumours that cannot be controlled with single-target agents [12].

Another group developed small molecules with dual-acting topoisomerase II (Topo II)–HDAC inhibitory activities. They found that many of these hybrid compounds more potently inhibited HDAC and Topo II activities compared to SAHA and daunomycin – standard HDACi and Topo II inhibitors, respectively. Additionally, a subset of these compounds exhibited potent whole-cell anti-proliferative activities against representative cancer cell lines [13]. More recently, the same group designed and synthesised dual-acting topoisomerase I (Topo I)–HDAC inhibitors, which displayed potent anti-proliferative activity as well [14]. Other investigators developed novel bifunctional platinum–HDACi conjugates as anticancer therapies. These drug candidates were shown to be highly cytotoxic as well as displaying enhanced selectivity for cancer cells over normal cells. In vivo experiments using a conjugate of Pt(IV)–HDAC inhibitor, VAAP, showed significant inhibitory effect on tumour growth and low systemic toxicity [15-17]. Some researchers chose to synthesise dual inhibitors of inosine monophosphate dehydrogenase and HDACs for the treatment of cancer. They reported that these compounds were more potent than the parent compounds as anti-proliferative agents. Further, these dual-acting inhibitors were significantly more potent with regard to inducing differentiation [18]. Finally, several promising HDACi-based compounds were synthesised that incorporated structural elements of ribonucleotide reductase or 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors in them [19,20].

2. Expert opinion

Owing to the heterogeneous and dynamic nature of tumours, the cancer medicine of the future will integrate, within a single small molecule, elements that allow for simultaneous targeting of multiple oncogenic targets, while maintaining potent anticancer activity and lower side effects. It is rational to hypothesise that balanced simultaneous modulation of a few druggable targets may have superior efficacy and fewer side effects than single-target or combination therapies for the treatment of human cancers. First, hybrid drugs with multiple inhibitory activities are expected to offer the advantage of pharmacokinetic simplicity that is lacking in combination therapies; nevertheless, the possibility that such drugs could lead to more pharmacokinetic complexity cannot be excluded at present. Second, such chimeric therapeutics is potentially able to overcome clinical resistance to single-agent therapy through counteracting the various mechanisms devised by cancer cells for escaping cell death by chemotherapeutic agents.

Interestingly, the broad capacity of HDACi for anti-tumour synergy may suggest that this class of drugs could be conjugated to a vast array of other anticancer agents to yield compounds with higher response rates. Researchers have already begun to design and synthesise novel HDACi-based hybrids in order to investigate the prospect of such agents in cancer therapy. Initial efforts were mainly concentrated on inhibition of proteins that belong to the same gene family, that is, simultaneous inhibition of two different HDAC enzymes. Theoretically, multi-targeting can be achieved by simply conjugating various HDACi pharmacophores, either isoenzyme selective or pan-HDACi, to other complementary chemo-active groups. Owing to the presence of many hydrophobic residues at the HDAC external cavities, it is plausible that appropriate conjugation of the HDACi surface recognition cap to other hydrophobic anti-tumour pharmacophores could furnish a new class of hybrid drugs. By exploiting this flexibility of HDACi, researchers can expand the repertoire of such hybrid drugs and develop broad-acting, effective anticancer agents. Apart from the examples of HDACi-based hybrid drugs mentioned here, other attractive starting points for a secondary target are enzymes that regulate the methylation state of DNA and histone proteins, such as DNA methyltransferases, histone methyltransferases and histone demethylases, proteins containing bromodomains that recognise and bind to acetylated residues, the proteasome and the retinoic acid receptor. The aforementioned preclinical studies are useful because they all provide a framework to create small molecules that concurrently tackle multiple oncogenic targets, indicating a general paradigm to surpass conventional, single-target cancer therapeutics and to significantly enhance therapeutic outcomes in many cancers, including solid malignancies.

Based on the in vitro testing of the cited examples, HDACi-derived hybrids appear to be more efficacious than the parent drug alone or in combination with two drugs. These hybrid molecules are a remarkable achievement in the field of drug development, since HDACi activity has fully integrated into the structure of several other therapeutic agents; rational design and skilful synthesis generated hybrids that effectively modulate both HDACs and their additional targets. Researchers who are interested in further exploring the versatility of this emerging strategy could discover surprising combinations of anticancer pharmacophores that may give rise to unexpected therapeutic benefits in the near future. Ongoing efforts to convert HDACi into potent bifunctional molecules will further validate the idea of using a single compound directed against multiple targets. Even if the integration of two anti-tumour pharmacophores results in a loss of potency for one of the two targets, incorporation of anti-tumour activity against a second, sympathetic biochemical target is expected to increase clinical efficacy.

Considering the possibility that such dual-acting compounds might be used in the oncology clinic in the near future, effective therapy requires that both anti-tumour pharmacophores demonstrate strong and balanced affinity for their respective target. Additionally, this balanced activity at each target of interest should be accompanied by a wider selectivity and a favourable pharmacokinetic profile [8]. In terms of the intracellular fate of the conjugate, investigations are needed to reveal whether stable or releasable conjugates are more potent anticancer agents. Releasable conjugate compounds have demonstrated anti-mitotic and pro-apoptotic activities with a reduced potency, while stable conjugates displayed a high cytotoxic potency [21]. The clinical use of HDACi-based conjugate drugs requires a better support by in vivo models to show and optimise the therapeutic benefit of these multivalent single drugs and, at the same time, to uncover any possible clinical limitations.

Increasingly over the coming decade, pharmacological strategies for developing multi-target anti-tumour compounds will be influenced by our rapidly evolving knowledge of the aberrant signalling pathways within human cancer cells and will be informed by highly accurate molecular diagnoses. The known genetic plasticity of tumour cell populations demands rational and complex strategies. The chimeric drugs cited here are rationally designed to synergistically suppress multiple oncogenic targets and represent the forerunners of a series of hybrid anticancer agents yet to come. Preclinical results indicate that such drugs hold great promise as candidates for future clinical development and have stimulated intense studies, which aim to overcome tumour relapse, metastasis and drug resistance. Currently, the therapeutic advantages, as well as limitations, of HDACi-based hybrid compounds on human cancer cells remain to be documented by in vivo studies.

Declaration of interest

The authors state no conflict of interest and have received no payment in preparation of this manuscript.